This invention relates to semiconductor bus switches, and more particularly to bi-directional undershoot protection for a MOS bus switch.
Bus switches are often used in networking applications. Bus switches using metal-oxide-semiconductor (MOS) technology have low on resistance, reducing delay through the switch. The source and drain nodes of a bus-switch transistor connect to the busses while the gate is controlled by a bus-connecting enable signal. See for example Parallel Micro-Relay Bus Switch for Computer Network Communication with Reduced Crosstalk and Low On-Resistance using Charge Pumps, U.S. Pat. No. 5,808,502, and Bus Switch Having Both P- and N-Channel Transistors for Constant Impedance Using Isolation Circuit for Live-Insertion when Powered Down U.S. Pat. No. 6,034,553.
More complex networks are emerging. For example, the bus switch may connect two processor buses. Each processor bus can operate independently of the other. Hot-plugging or hot-swapping of card with the processor bus can also occur. When the bus switch is in the isolation mode, full isolation must occur, regardless of which bus is active.
FIG. 1 shows a typical application of a bus switch. First local bus signals 18 (bus A) is connected to CPU_A 10, memory_A 14, and Application-Specific Integrated Circuit (ASIC_A) 12. Second local bus signals 19 (bus B) is a second local bus that has CPU_B 11, memory_B 15, and Application-Specific Integrated Circuit (ASIC_B) 13. Second local bus signals 19 is a hot-plugable bus. Switch network 16 connects address, data, and control lines from bus signals 18 to bus signals 19 using MOS transistors. One transistor is used for each bus signal.
When a device is plugged into bus signals 19, it may be desired to isolate bus signals 19 from local bus signals 18. Noise caused by the plugging operation can then be isolated to bus signals 19, allowing local bus signals 18 to operate unhindered. Switch network 16 can isolate bus signals 19 from local bus signals 18 by applying a low voltage to n-channel transistors in switch network 16. When switch network 16 isolates, Bus_A can operate independently of Bus_B.
Either Bus_A or Bus_B may be hot-plugged into the other bus. This allows for repair of systems without any downtime. Isolation by switch network 16 must therefore be fully bi-directional since it is not known which bus will be replaced until a failure occurs.
Undershoot Problem
When an n-channel transistor is used as the bus switch, the bus switch is disabled by driving a ground voltage to the gate of the n-channel bus-switch transistor. The output bus signal should be isolated from voltage changes at the input bus signal. The quality of the signal waveforms on local bus signal 18 is not always well controlled. Sometime large voltage spikes below ground (undershoots) occur, especially on the high-to-low transitions from high-current drivers on local bus signal 18. The same could occur on bus signals 19.
When the bus-switch input from bus signal 18 goes below ground, a positive gate-to-source voltage develops on bus-switch transistor since its gate is at ground. A conducting channel forms below the gate. When the undershoot is greater than a volt, this gate-to-source voltage exceeds the n-channel threshold voltage, turning on the n-channel bus switch transistor. Some current is conducted through the channel of the bus-switch transistor even though its gate may be kept at ground. The result is that the voltage is disturbed on the drain of the bus-switch transistor, and the output to bus signals 19.
When the source of the n-channel bus-switch transistor goes negative during the undershoot, the base-emitter junction of the parasitic lateral NPN transistor is forward biased, coupling more current to the output through the p-type substrate.
The result of the undershoot is that the output connects to the input for a short period of time, the duration of the undershoot. The voltage on the drain of the bus-switch transistor can quickly fall from the power supply (Vcc) to ground and even below ground should the undershoot last for more than a few nanoseconds. The undershoots on the input bus coupled to the output, producing severe voltage disturbances on the isolated bus.
The inventor has solved an undershoot-isolation problem in earlier patents, such as U.S. Pat. No. 6,052,019 for Undershcot-lsolating MOS Bus Switch. However, this patent shows a circuit that is effective when the undershoot always occurs on only one side of the bus switch. Another improved circuit using a pulse generator was shown by the inventor in "Bi-Directional Undershoot-Isolating Bus Switch with Directional Control", U.S. Ser. No. 09/607,460, filed Jun. 29, 2000. While useful, a fully bi-directional undershoot-isolating bus switch without the pulse generator is desired.